Optical fibers generally have a diameter of about 100 .mu.m and, hence, their light transmission performances tend to be adversely affected by stresses due to impact from outside and linear expansion. When, therefore, the optical fibers are used in electric cables, for example, they have been coated with FRP to increase their mechanical strength and reliability. In this case, it is required for the FRP to have not only a high toughness but also a high heat resistance which can withstand a temporary high temperature condition (300.degree.-500.degree. C.) due to the lightning and the formation of surge current.
However, thermoplastic resins such as a polyethylene terephthalate or a polubutylene phthalate and thermosetting resins such as an epoxy resin or an unsaturated polyester resin which have been used in production of conventional FRP are easily thermally decomposed at the above-described high temperatures.
Various resins have been proposed as resin starting materials for producing the heat resistant FRP. FRP produced using these resins, however, do not satisfy the above two requirements, heat resistance and toughness. For example, if compounds containing a heterocyclic ring such as an isocyanurate ring are blended to increase the heat resistance, the heat resistance of the resulting FRP increases, but its toughness decreases. Thus, they cannot be used satisfactorily to coat the optical fibers. Furthermore, since the resins are often used in combination with solvents and their pot lives are short, they suffer from a disadvantage in that workability for coating the optical fibers is poor.